The use of flexible containers for the containment and transportation of cargo, particularly fluid or liquid cargo, is well known. It is well known to use containers to transport fluids in water, particularly, salt water.
If the cargo is fluid or a fluidized solid that has a density less than salt water, there is no need to use rigid bulk barges, tankers or containment vessels. Rather, flexible containment vessels may be used and towed or pushed from one location to another. Such flexible vessels have obvious advantages over rigid vessels. Moreover, flexible vessels, if constructed appropriately, allow themselves to be rolled up or folded after the cargo has been removed and stored for a return trip.
Throughout the world there are many areas which are in critical need of fresh water. Fresh water is such a commodity that harvesting of the ice cap and icebergs is rapidly emerging as a large business. However, wherever the fresh water is obtained, economical transportation thereof to the intended destination is a concern.
For example, currently an icecap harvester intends to use tankers having 150,000 ton capacity to transport fresh water. Obviously, this involves, not only the cost involved in using such a transport vehicle, but the added expense of its return trip, unloaded, to pick up fresh cargo. Flexible container vessels, when emptied can be collapsed and stored on, for example, the tugboat that pulled it to the unloading point, reducing the expense in this regard.
Even with such an advantage, economy dictates that the volume being transported in the flexible container vessel be sufficient to overcome the expense of transportation. Accordingly, larger and larger flexible containers are being developed. However, technical problems with regard to such containers persist even though developments over the years have occurred. In this regard, improvements in flexible containment vessels or barges have been taught in U.S. Pat. Nos. 2,997,973; 2,998,973; 3,001,501; 3,056,373; and 3,167,103. The intended uses for flexible containment vessels is usually for transporting or storing liquids or fluidisable solids which have a specific gravity less than that of salt water.
The density of salt water as compared to the density of the liquid or fluidisable solids reflects the fact that the cargo provides buoyancy for the flexible transport bag when a partially or completely filled bag is placed and towed in salt water. This buoyancy of the cargo provides flotation for the container and facilitates the shipment of the cargo from one seaport to another.
In U.S. Pat. No. 2,997,973, there is disclosed a vessel comprising a closed tube of flexible material, such as a natural or synthetic rubber impregnated fabric, which has a streamlined nose adapted to be connected to towing means, and one or more pipes communicating with the interior of the vessel such as to permit filling and emptying of the vessel. The buoyancy is supplied by the liquid contents of the vessel and its shape depends on the degree to which it is filled. This patent goes on to suggest that the flexible transport bag can be made from a single fabric woven as a tube. It does not teach, however, how this would be accomplished with a tube of such magnitude. Apparently, such a structure would deal with the problem of seams. Seams are commonly found in commercial flexible transport bags, since the bags are typically made in a patch work manner with stitching or other means of connecting the patches of water proof material together. See e.g. U.S. Pat. No. 3,779,196. Seams are known to be a source of bag failure when the bag is repeatedly subjected to high loads. Seam failure can obviously be avoided in a seamless structure.
Other problems face the use of large transport containers. In this regard, when partially or completely filled flexible barges or transport containers are towed through salt water, problems as to instability are known to occur. This instability is described as a flexural oscillation of the container and is directly related to the flexibility of the partially or completely filled transport container. This flexural oscillation is also known as snaking. Long flexible containers having tapered ends and a relatively constant circumference over most of their length are known for problems with snaking. Snaking is described in U.S. Pat. No. 3,056,373, observing that flexible barges having tapered ends, build up to damaging oscillations capable of seriously rupturing or, in extreme cases, destroying the barge, when towed at a speed above a certain critical speed. Oscillations of this nature were thought to be set up by forces acting laterally on the barge towards its stern. A solution suggested was to provide a device for creating breakaway in the flow lines of the water passing along the surface of the barge and causing turbulence in the water around the stern. It is said that such turbulence would remove or decrease the forces causing snaking, because snaking depends on a smooth flow of water to cause sideways movement of the barge.
Other solutions have been proposed for snaking by, for example, U.S. Pat. Nos. 2,998,973; 3,001,501; and 3,056,373. These solutions include drogues, keels and deflector rings, among others.
Another solution for snaking is to construct the container with a shape that provides for stability when towing. A company known as Nordic Water Supply located in Norway has utilized this solution. Flexible transport containers utilized by this company have a shape that can be described as an elongated hexagon. This elongated hexagon shape has been shown to provide for satisfactory stable towing when transporting fresh water on the open sea. However, such containers have size limitations due to the magnitude of the forces placed thereon. In this regard, the relationship of towing force, towing speed and fuel consumption for a container of given shape and size comes into play. The operator of a tugboat pulling a flexible transport container desires to tow the container at a speed that minimizes the cost to transport the cargo. While high towing speeds are attractive in terms of minimizing the towing time, high towing speeds result in high towing forces and high fuel consumption. High towing forces require that the material used in the construction of the container be increased in strength to handle the high loads. Increasing the strength typically is addressed by using thicker container material. This, however, results in an increase in the container weight and a decrease in the flexibility of the material. This, in turn, results in an increase in the difficulty in handling the flexible transport container, as the container is less flexible for winding and heavier to carry.
Moreover, fuel consumption rises rapidly with increased towing speed. For a particular container, there is a combination of towing speed and fuel consumption that leads to a minimum cost for transportation of the cargo. Moreover, high towing speeds can also exacerbate problems with snaking.
In the situation of the elongated hexagon shaped flexible transport containers used in the transport of fresh water in the open sea, it has been found, for a container having a capacity of 20,000 cubic meters, to have an acceptable combination of towing force (about 8 to 9 metric tons), towing speed (about 4.5 knots) and fuel consumption. Elongated hexagon shaped containers having a capacity of 30,000 cubic meters are operated at a lower towing speed, higher towing force and higher fuel consumption than a 20,000 cubic meter cylindrical container. This is primarily due to the fact that the width and depth of the larger elongated hexagon must displace more salt water when pulled through open sea. Further increases in container capacity are desirable in order to achieve an economy of scale for the transport operation. However, further increases in the capacity of elongated hexagon shaped containers will result in lower towing speeds and increased fuel consumption.
The aforenoted concerning snaking, container capacity, towing force, towing speed and fuel consumption defines a need for an improved flexible transport container design. There exists a need for an improved design that achieves a combination of stable towing (no snaking), high FFCV capacity, high towing speed, low towing force and low fuel consumption relative to existing designs.
In addition, to increase the volume of cargo being towed, it has been suggested to tow a number of flexible containers together. Such arrangements can be found in U.S. Pat. Nos. 5,657,714; 5,355,819; and 3,018,748 where a plurality of containers are towed in line one after another. So as to increase stability of the containers, EPO 832 032 B1 discloses towing multiple containers in a pattern side by side.
However, in towing flexible containers side by side, lateral forces caused by ocean wave motion creates instability which results in one container pushing into the other and rolling end over end. Such movements have a damaging effect on the containers and also effect the speed of travel.
Another problem with such flexible containers is the large towing forces thereon, in addition to the forces created by extreme sea and wind conditions. Accordingly, it is imperative that ruptures in the container be avoided, otherwise the entire cargo could become compromised. Reinforcing the container against such failures is desirable and various means for reinforcing the container have been proposed. These typically include the attachment of ropes to the outer surface of the container, as can be seen in, for example, U.S. Pat. Nos. 2,979,008 and 3,067,712. Reinforcement strips and ribs cemented to the outer surface of the container have also been envisioned, as disclosed in U.S. Pat. No. 2,391,926. Such reinforcements, however, suffer the disadvantages of requiring their attachment to the container while also being cumbersome, especially if the container is intended to be wound up when emptied. Moreover, external reinforcements on the container's surface provide for increased drag during towing. While reinforcements are very desirable, especially if a somewhat light weight fabric is envisioned, the manner of reinforcement needs to be improved upon.
Furthermore, while as aforenoted, a seamless flexible container is desirable and has been mentioned in the prior art, the means for manufacturing such a structure has its difficulties. Heretofore, as noted, large flexible containers were typically made in smaller sections which were sewn or bonded together. These sections had to be water impermeable. Typically such sections, if not made of an impermeable material, could readily be provided with such a coating prior to being installed. The coating could be applied by conventional means such as spraying or dip coating.
For larger coated fabrics (i.e. 40′×200′), it is possible to coat them using a large two roll liquid coating system. Although large, these fabrics are not as large as required for FFCVs. It is economically impractical to build a roll system to coat a fabric of the large size envisioned.
As distinct from the roll system, impermeable fabrics have also traditionally been made by applying a liquid coating to a woven or non-woven base structure and then curing or setting the coating via heat or a chemical reaction. The process involves equipment to tension and support the fabric as the coating is being applied and ultimately cured. For fabrics in the size range of 100″ in width, conventional coating lines are capable of handling many hundreds or thousands of feet. They involve the use of support rolls, coating stations and curing ovens that will handle woven substrates that fall within the 100″ width.
However, with an extremely large flexible woven seamless container, in order of 40′ diameter and 1000′ in length or larger, conventional coating methods would be difficult. While relatively small flat fabrics are readily coated, a tubular unitary structure, extremely long and wide, is much more difficult.
Accordingly, there exist a need for a FFCV for transporting large volumes of fluid which overcomes the aforenoted problems attendant to such a structure and the environment in which it is to operate.